TireSizesFinder

245/45R21 vs 285/40R21

285/40R21 is 0.30 in (7.5 mm) taller than 245/45R21, a +0.99% diameter difference. With the speedometer set for 245/45R21, an indicated 60 mph is 60.6 mph on 285/40R21.

Is this a safe swap?

Diameter difference +0.99%: under 1%

By overall diameter alone, 285/40R21 is a near match for 245/45R21; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 245/45R21 (29.68 in) vs 285/40R21 (29.98 in)245/45R2129.68 in × 9.65 in285/40R2129.98 in × 11.22 in
Side profile and tread width, drawn to the same scale.
245/45R21 vs 285/40R21 differences
Measurement245/45R21285/40R21Difference% diff
Overall diameter29.68 in (753.9 mm)29.98 in (761.4 mm)+0.30 in (+7.5 mm)+0.99%
Sidewall height4.34 in (110.3 mm)4.49 in (114.0 mm)+0.15 in (+3.8 mm)+3.40%
Section width9.65 in (245.0 mm)11.22 in (285.0 mm)+1.57 in (+40.0 mm)+16.33%
Circumference93.25 in (2,368.4 mm)94.17 in (2,392.0 mm)+0.93 in (+23.6 mm)+0.99%
Revolutions per mile679673−7−0.99%
True speed with the speedometer calibrated for 245/45R21
IndicatedActual on 245/45R21Actual on 285/40R21Difference
30 mph30.0 mph30.3 mph+0.3 mph
60 mph60.0 mph60.6 mph+0.6 mph
75 mph75.0 mph75.7 mph+0.7 mph

Ride-height change: 285/40R21 raises the vehicle by 0.15 in (3.8 mm) at the axle (half the +7.5 mm diameter difference).

285/40R21 is 0.30 in taller than 245/45R21 and 40 mm wider, a 0.99% difference in overall diameter.

With the speedometer calibrated for 245/45R21, an indicated 60 mph is actually 60.6 mph on 285/40R21, and revolutions per mile change from 679 to 673.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 245/45R21

Other comparisons with 285/40R21

Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.